Because a nitride semiconductor is featured in having a high breakdown voltage and high carrier mobility, utilization of the nitride semiconductor as a power device, etc. is expected.
A field effect transistor used in a power device, etc. and made of a nitride semiconductor is demanded to have high reliability particularly in an off-state where a high voltage is applied, namely to have a long lifetime.
One example of related-art field effect transistors has a structure in which a substrate withstand voltage is improved with control of a carbon concentration in a GaN substrate, as disclosed in Japanese Unexamined Patent Application Publication No. 2010-245504 (PTL 1).